Int. J. Mol. Sci. 2012, 13, 11288-11311; doi:10.3390/ijms130911288 OPEN ACCESS International Journal of Molecular Sciences ISSN 1422-0067 www.mdpi.com/journal/ijms Review Asymmetric Dimethylarginine, Endothelial Dysfunction and Renal Disease Luis Aldámiz-Echevarría * and Fernando Andrade Division of Metabolism, Cruces University Hospital, Barakaldo, Basque Country 48903, Spain; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel./Fax: +34-94-600-6327. Received: 2 August 2012; in revised form: 28 August 2012 / Accepted: 3 September 2012 / Published: 10 September 2012 Abstract: L-Arginine (Arg) is oxidized to L-citrulline and nitric oxide (NO) by the action of endothelial nitric oxide synthase (NOS). In contrast, protein-incorporated Arg residues can be methylated with subsequent proteolysis giving rise to methylarginine compounds, such as asymmetric dimethylarginine (ADMA) that competes with Arg for binding to NOS. Most ADMA is degraded by dimethylarginine dimethyaminohydrolase (DDAH), distributed widely throughout the body and regulates ADMA levels and, therefore, NO synthesis. In recent years, several studies have suggested that increased ADMA levels are a marker of atherosclerotic change, and can be used to assess cardiovascular risk, consistent with ADMA being predominantly absorbed by endothelial cells. NO is an important messenger molecule involved in numerous biological processes, and its activity is essential to understand both pathogenic and therapeutic mechanisms in kidney disease and renal transplantation. NO production is reduced in renal patients because of their elevated ADMA levels with associated reduced DDAH activity. These factors contribute to endothelial dysfunction, oxidative stress and the progression of renal damage, but there are treatments that may effectively reduce ADMA levels in patients with kidney disease. Available data on ADMA levels in controls and renal patients, both in adults and children, also are summarized in this review. Keywords: asymmetric dimethylarginine (ADMA); arginine (Arg); children; dimethylarginine dimethylaminohydrolase (DDAH); endothelial dysfunction; kidney; methylarginines; nitric oxide; oxidative stress; renal failure Int. J. Mol. Sci. 2012, 13 11289 Abbreviations: ADMA, asymmetric dimethylarginine; Arg, L-arginine; cGMP, cyclic guanosine monophosphate; Citr, citrulline; cNOS, constitutive nitric oxide synthase; CRP, C reactive protein; DDAH, dimethylarginine dimethylaminohydrolase; eNOS, endothelial nitric oxide synthase; GFR, glomerular filtration rate; Hcys, homocysteine; iNOS, inducible oxide nitric synthase; LDL, low density lipoproteins; NMMA, N-monomethyl-L-arginine; nNOS, neuronal oxide nitric synthase; NO, nitric oxide; NOS, nitric oxide synthase; PRMT, protein arginine methyltransferase; RTx, renal transplantation; SDMA, symmetric dimethylarginine. 1. Introduction In addition to the traditional cardiovascular risk factors, other factors associated with renal failure involved in common metabolic pathways have recently been studied in detail. The metabolism of amino acids tends to be impaired in patients with chronic renal failure due to abnormalities in their synthesis or excretion. For example, one of the alterations of amino acid metabolism in patients with chronic renal failure is hyperhomocysteinaemia; this is considered to be an independent risk factor for cardiovascular disease [1] and occurs in more than 85% of patients after kidney transplant (RTx) and in those with end-stage renal failure [2]. The new metabolic pathways currently being described may help, to some extent, to group these conditions by pathophysiological characteristics and to identify possible alternative therapeutic approaches. In particular, it seems interesting to study the metabolism of arginine and associated metabolic pathways. For this, it is useful to determine the levels of arginine and its methylated derivatives as part of the analysis of the cardiovascular risk and endothelial dysfunction of renal disease. 2. Nitric Oxide and the Kidney The vascular endothelium has many functions and, accordingly, endothelial dysfunction is responsible for numerous health problems including atherosclerosis, high blood pressure, sepsis, thrombosis, vasculitis, and bleeding, among others. One of the most important functions of the endothelium is to secrete nitric oxide (NO), a relatively unstable diatomic free radical, which can be synthesized by a broad range of organisms. This molecule has a role as a messenger in many biological processes in humans including involvement in the regulation of neurone communication, antimicrobial activity, ventilation, hormone secretion, inflammation and immune responses well as vascular tone [3]. Indeed, it is a potent vasodilator and levels are often reduced when endothelial function is impaired, making it a vascular risk factor and, in particular, together with dyslipidaemia, a risk factor for coronary disease. Nitric oxide was first described as an endothelium-derived vascular relaxant factor, but its role in vasodilation depends on an increase in the levels of cyclic guanosine monophosphate (cGMP) in smooth muscle cells (Figure 1). In this case, NO is synthesized from arginine by the enzyme nitric oxide synthase (NOS), which has been identified in neurons, endothelial cells, macrophages and hepatocytes in various different isoforms: inducible (iNOS) and constitutive (cNOS), which contains neuronal (nNOS), and endothelial (eNOS). Int. J. Mol. Sci. 2012, 13 11290 Figure 1. Nitric oxide (NO) synthesis in vascular endothelium and its diffusion to smooth-muscle cells where soluble guanylyl cyclise (sGC) is stimulated resulting in enhanced synthesis of cyclic guanosine monophosphate (GMP). In the kidney, where the synthesis of arginine occurs mainly in the proximal tubules, cNOS has been found in the glomerules, vessels and tubular segments including the macula densa and inner medullary segments of the collecting duct system [4,5]. The iNOS isoform has been found in the smooth muscle cells of blood vessels, the distal end of the efferent arteriole and the medullary area of the ascending limb of the loop of Henle [6]. Cytokines that stimulate this inducible form (iNOS) have been found in cultures from proximal tubules, inner medullary segments of the collecting duct system and the mesangium [7]. Renal medullary blood flow varies in response to endothelium-dependent vasodilation and endothelial cells of the vasa recta renis are capable of producing NO, which can have an influence on transport within the collecting duct system. The inhibition of NO synthesis in the kidney may have numerous consequences for patients: Reduced glomerular blood flow, together with an increase in the vascular resistance of the afferent and efferent arterioles; Reduced ultrafiltration, renal blood flow and glomerular filtration rate (GFR); Decreased secretion of renin, a hormone involved in the sodium and water balance in the body; Reduced ability to excrete sodium under normal conditions; Increased blood pressure and deterioration in renal function; + - Lack of stimulation for Na and HCO3 transport in the nephron proximal tubules mediated by cGMP; Production of oxygen reactive species; Production of nitric peroxide when exposed to superoxide anions. In brief, it has relatively recently been discovered that NO is an important messenger molecule involved in numerous biological processes, and knowledge about its activity is essential to help us understand both pathogenic and therapeutic mechanisms in kidney disease. Int. J. Mol. Sci. 2012, 13 11291 2.1. Endothelial Dysfunction and Oxidative Stress in Kidney Disease Several theories indicate that endothelial dysfunction is what predisposes individuals to rapid atherosclerosis that, in turn, is involved in the pathogenesis of high blood pressure and associated with renal function decline [8]. On the other hand, it has been suggested that a reduction in NOS activity or an increase in its degradation rate, due to oxidative stress, may cause endothelial damage [9]. Oxidative stress in patients who have undergone RTx is mainly due to endothelial dysfunction caused by the inhibition of NO synthesis. It should, however, be taken into account that the use of immunosuppressants also influences oxidative stress in patients after RTx, since cyclosporine and tacrolimus cause post-transplant hypertension, which weakens defenses against oxidative stress [10,11]. In the case of tracrolimus, it has been described how the immunosuppressant can cause inhibition of the activity and transcription of eNOS [12], leading to a reduction in NO levels. Reduced production of endothelial NO could also be a direct consequence of therapy with cyclosporine since it induces calciuria, and this would increase the risk of early atherothrombosis in patients after RTx [13]. It is very important to study the role of arginine in this type of patient, since NOS activity in kidney failure is determined by arginine concentration [14], the key role of the kidney in the production of the NO precursor making renal patients prone to NO deficiency. 2.2. Arginine-Nitric Oxide Metabolism L-arginine (Arg) can be broken down by several metabolic pathways: apart from its transformation into guanidinoacetate and creatine, it can be oxidized to L-citrulline (Citr) and NO by endothelial NOS (its main substrate) (Figure 2a). In line with this, acute or chronic administration of